1. Field of the Invention
The invention concerns a protective device for at least one thyristor.
2. Description of the Prior Art
Thyristors can be destroyed by overvoltages which exceed the permissible peak voltage of the thyristor or by an excessive rate of rise of the voltage. External overvoltage circuits, for instance, with overvoltage arresters and RLC members are in the prior art for the purpose of protecting thyristors, such as are described, for instance, in the book "Thyristor Handbook," page 193 to 195.
These external overvoltage circuits are particularly important for thyristor switches for high-voltage applications, i.e., in HGU installations. These thyristor switches are formed by connecting in series an appropriate number of thyristors, which are controlled by means of magnetic pulse transformers.
These thyristor switches can be subjected to an overvoltage stress at any time by external causes such as flashovers at insulators or by lightning strokes hitting the transmission lines. Extensive protection of the switches against high voltages is achieved with an external overvoltage circuit mentioned above, in which the switches are shunted by overvoltage arresters which limit the magnitude of the overvoltage to permissible values for the thyristors and in which the rate of rise of the voltage for the thyristors within a switch is limited by the use of RCL members which are arranged distributed in the switch.
Only if overvoltages occur during a certain time interval, i.e., during the recovery of the thyristors which follows the current conduction time, can renewed firing of each thyristor occur if a positive voltage should appear at its anode-cathode path at a specific voltage value. This voltage value which can renew firing depends on the electric stress and the individual thyristor unit. It should further be mentioned here that the recovery time of a thyristor increases with the rate of rise and with the magnitude of the anode voltage from negative toward positive values. Renewed switching on of thyristors by means their anode-cathode voltage without a control pulse, so-called "high-level firing," may lead to the destruction of thyristors, which cannot be avoided completely with known external overvoltage circuits.
It is an object of this invention to provide a protective device such that high-level firing of the thyristor is impossible is overvoltages occur during the recovery time of the thyristor.